The simple multicellular organism Volvox is being used for studies on the control of cellular differentiation. Volvox carteri f. nagariensis is the main object of investigation although other species are being used at times for comparative purposes. The chemical and physical nature of the sexual hormone and its action in controlling, i.e., initiating, the sexual pattern of development in the embryos, are being studied. Evidence indicates the presence of a small molecular weight molecule in addition to the larger one (30,000 MW) in a new recombinant male filtrate; attempts will be made to isolate and characterize this small molecule during the coming year. Further work will be done on mutants which show decreased response to the sexual hormone in an effort to relate this action to other biochemical systems in the gonidia.